Rheumatic fever and rheumatic heart disease remain as important causes of cardiovascular disease. The ultimate control of this theoretically preventable disease requires control of preceding streptococcal upper respiratory tract infections. However, important unanswered questions about the epidemiology of streptococcal infections and especially about individual susceptibility to these infections require answers. This study is designed to address these questions. In addition to studying the epidemiology of streptococcal respiratory tract infection (concentrating on the risk of intra-familial spread), this study will evaluate new diagnostic techniques and validate and correlate older techniques used in the diagnosis of this infection. Since it is thought that the carrier of group A streptococci presents both a smaller risk to himself (in terms of developing non-suppurative sequelae of group A streptococci) and to his contacts (less likely to spread the organism to others), it is essential that better techniques be devised to allow clinical differentiation between bona fide group A streptococcal infection and the symptomatic or asymptomatic carrier. This information may ultimately lead to a reduction in the incidence of rheumatic fever and thereby reduce the morbidity and mortality from rheumatic heart disease. The third year of the study will be carried out among index cases of pharyngitis and their family contacts seen in a clinic-emergency room setting. Clinical observations and both established and new laboratory techniques will be serially obtained and correlated to assist in identifying and documenting host and bacterial risk factors affecting the epidemiology of streptococcal infections and in differentiating bona fide streptococcal infections from the carrier state.